BUG: fix regression in Styler.export/use (#40334)

Author: attack68

doc/source/whatsnew/v1.3.0.rst

@@ -613,7 +613,7 @@ Other
 - Bug in :func:`pandas.api.types.infer_dtype` not recognizing Series, Index or array with a period dtype (:issue:`23553`)
 - Bug in :func:`pandas.api.types.infer_dtype` raising an error for general :class:`.ExtensionArray` objects. It will now return ``"unknown-array"`` instead of raising (:issue:`37367`)
 - Bug in constructing a :class:`Series` from a list and a :class:`PandasDtype` (:issue:`39357`)
-- Bug in :class:`Styler` which caused CSS to duplicate on multiple renders. (:issue:`39395`)
+- Bug in :class:`Styler` which caused CSS to duplicate on multiple renders. (:issue:`39395`, :issue:`40334`)
 - ``inspect.getmembers(Series)`` no longer raises an ``AbstractMethodError`` (:issue:`38782`)
 - Bug in :meth:`Series.where` with numeric dtype and ``other = None`` not casting to ``nan`` (:issue:`39761`)
 - :meth:`Index.where` behavior now mirrors :meth:`Index.putmask` behavior, i.e. ``index.where(mask, other)`` matches ``index.putmask(~mask, other)`` (:issue:`39412`)

pandas/io/formats/style.py

@@ -855,10 +855,10 @@ def _compute(self):
 
         (application method, *args, **kwargs)
         """
+        self.ctx.clear()
         r = self
         for func, args, kwargs in self._todo:
             r = func(self)(*args, **kwargs)
-        self._todo = []
         return r
 
     def _apply(

pandas/tests/io/formats/style/test_style.py

@@ -179,11 +179,10 @@ def test_clear(self):
         assert len(s.cell_context) > 0
 
         s = s._compute()
-        # ctx and _todo items affected when a render takes place
+        # ctx item affected when a render takes place. _todo is maintained
         assert len(s.ctx) > 0
-        assert len(s._todo) == 0  # _todo is emptied after compute.
+        assert len(s._todo) > 0
 
-        s._todo = [1]
         s.clear()
         # ctx, _todo, tooltips and cell_context items all revert to null state.
         assert len(s.ctx) == 0
@@ -767,7 +766,7 @@ def test_export(self):
         f = lambda x: "color: red" if x > 0 else "color: blue"
         g = lambda x, z: f"color: {z}" if x > 0 else f"color: {z}"
         style1 = self.styler
-        style1.applymap(f).applymap(g, z="b").highlight_max()
+        style1.applymap(f).applymap(g, z="b").highlight_max()._compute()  # = render
         result = style1.export()
         style2 = self.df.style
         style2.use(result)